1. Field of Invention
The present invention relates to optical communications systems. More particularly, the present invention relates to polarization mode dispersion compensating systems and methods.
2. Description of Related Art
Polarization mode dispersion (PMD) is a well-known problem caused by the undesired, residual birefringent properties of optical fibers. Despite the attempts of optical fiber manufacturers to eliminate PMD, a residual amount of PMD is still present. Moreover, there are a variety of existing fiber plants having fibers not optimized to reduce PMD.
Essentially, PMD causes the two principal states of polarization to propagate along an optical fiber at different rates. The polarization of an optical signal may be expressed in terms of two components (the so-called “principal states of polarization” (POS)). The two principal polarization states each experience different propagation delays as they propagate down a length of optical fiber due to the residual birefringence of the fiber. The result of the time delay (τ) between the two principal states (also referred to as differential group delay, or DGD) is that the signal is distorted. The time delay (τ) may be on the order of 10–20 ps for a 100 km fiber
More accurately, DGD exhibits a gaussian distribution. DGD values, such as the 10–20 ps mentioned above, are usually a mean value of this gaussian distribution. However, the Gaussian distribution means that there is a likelihood of a DGD value much larger than the mean. Such large DGD values can cause optical signal pulses (composed of both principal polarization states) to broaden to such an extent that intersymbol interference occurs and the bit error rate (BER) rises.
In addition, higher data rates and longer transmission distances make even small PMD values a more significant problem than in the past. For example, at 10 Gb/s only 100 ps separates each pulse. Thus, a 50 ps PMD could easily cause a bit error. The progression to 40 Gb/s and higher will make PMD compensation an important problem to solve.
Exacerbating these problems is the fact that PMD varies with time, fiber temperature, and fiber stress. For example, a technician moving a fiber will stress the fiber and induce fluctuating PMD. Temperature cycling will also cause a fluctuating PMD. Thus, a PMD compensator having a fixed amount of counter-PMD will not adequately offset the time-varying PMD.
To address these problems, various PMD compensation schemes have been invented. Many of these schemes employ variable time-delay elements that subject one of the two principal polarization states to a variable delay in order to align the phases of the principal states.
Fee et al. (U.S. Pat. No. 5,859,939) is an example of such a variable delay element approach. Fee's polarization beam splitter splits the input beam in order to detect the delay between the principal polarization states. Incremental delay elements made from different lengths of optical fiber are then switched into the optical path of one (or both) of the principal polarization states in order to align the phases thereof. The phase-compensated principal states are then combined, hopefully with a reduced PMD.
Hakki (U.S. Pat. No. 5,659,412) is another example of such a variable time-delay element approach. Hakki uses a polarization beam splitter to split the incoming signal into transverse electric (TE) and transverse magnetic (TM) polarized components after a polarization controller aligns the PSPs of the received optical signal with the polarization axes of the beam splitter. The TM component is delayed by a variable electrical delay element. A phase detection circuit measures a phase difference between the components and is used to control the variable electrical delay element and the polarization controller. The compensated components are then combined and fed to a receiver (decision circuit).